The present invention relates to electrochemical cells and, more particularly, to portable cells adapted for the detection and measurement of selected gases in the atmosphere.
The increasing concern with pollutants, in both the atmosphere and specfic breathing environments, has caused a sudden demand to supplement conventional laboratory apparatus for detecting and measuring components in a gaseous mixture, such as air, with more portable devices. For example, carbon monoxide has heretofore been measured using infrared techniques. Such apparatus is not readily adaptable for wear by mine workers or even placement on mine vehicles. Electrochemical techniques present a potential solution but not, however, without problems. Since the use of aqueous electrolytes is employed in electrochemical detection and measurement, there is a safety factor to be considered, particularly when the apparatus is to be worn on the person. Also, electrochemical reactions develop a current signal of very small magnitude when employed in a detector sized to be easily carried or worn. This can result in a requirement for larger heavier amplification means as well as causing an inherent lack of stability and response.
As described herein, the present invention is directed to the measurement of carbon monoxide. As will be understood by those skilled in the art, a cell of the type herein described could by used equally well in the detection of numerous gases and, alternatively, could be employed as a fuel cell.
Electrochemical determination of carbon monoxide is one of the most sensitive methods of detection. Electrodes used are formed from the noble metals, e.g. platinum or gold. The electrolyte used is usually sulfuric or phosphoric acid. In one apparatus, a suitable potential is applied between the anode (working electrode) and the cathode (counter electrode). This arrangement permits the amperometric detection of the resulting current flow between anode and cathode. In another commonly used technique, an additional (reference) electrode is employed. In this manner, the presence of carbon monoxide may be detected potentiostatically. In such apparatus, the reference electrode is a hydrogen electrode. Typical prior art cells include those of Niedrach et al. (U.S. Pat. No. 3,432,355) and Oswin et al. (U.S. Pat. No. 3,776,832). In the patent of Niedrach et al. and an article by Niedrach and Alford "A New High-Performance Fuel Cell Employing Conducting-Porous-Teflon Electrodes and Liquid Electrolytes, " Journal of the Electrochemical Society, Vol. 112, No. 2, Feb. 1965, page 117, the authors describe the construction of the electrodes used in their cell by the technique of forming a platinum black and polytetrafluoroethylene suspension on a polytetrafluroethylene membrane at an elevated temperature to cause the bonding of the polytetrafluoroethylene and platinum black. A platinum screen is then disposed between two such coated membranes with the membrane material on the outsides and the total assembly is sintered together at an elevated temperature, and preferably under pressure, to form a unitary element. According to both these prior art references, one surface of the electrode is exposed to the electrolyte and the opposite is exposed to the gas being tested. In order for the electrochemical reaction to take place between electrodes thus disposed, the electrolyte must disperse through the electrode or, alternatively, the working gas must disperse through the electrode. In either case, movement through the electrode is required and the surface available for electrochemical reaction per unit area of electrode is minimal because of the method of forming.
While the cell structures are somewhat identical in both cases, the cell of Niedrach et al. is directed to a fuel cell application while that of Oswin et al. is directed toward electrochemical detection of noxious gases in the atmosphere. A cell such as that described by Oswin et al. can only be operated in one position and is not, therefore, readily adpated for use as a personal monitor such as in mines, etc. Moreover, these prior art electrodes are not only sensitive to carbon monoxide but also NO, SO.sub.2, H.sub.2 S, unsaturated hydrocarbons and other organics. If the ambient air being tested contains gases other than pure reference gas, the observed reading due to carbon monoxide is decreased in relation to its sensitivity and concentration. The interference by these other gases occurs in two ways. When entering through the sensitive (inlet) side adjacent the anode, CO and the other gases to which the electrode will react cause a positive going signal depending upon their relative proportions. The same gases if allowed to come in contact with the other (counter) electrode will result in a signal of opposite polarity. This problem was realized by others who suggested that in apparatus employing a reference electrode such as that of Oswin et al., a reference atmosphere be provided by maintaining an atmosphere of pure air over the reference electrode. This technique also tends to make the detector not portable and, further, provides a sensor with a data output that is not correlatable with other known standard methods, such as infrared detection of carbon monoxide.
Thus, it is the object of the present invention to provide a truly portable electrochemical cell having a high current signal for its size capable of either two electrode amperometric detection or three electrode potentiostatic detection.